1. Field of the Invention
This invention relates to a new and unique method and apparatus for modulating the perceptible intensity of the presentation of a light emitting display. While a wide variety of light emitting displays are available, this invention is directed to those light emitting displays which include a display apparatus and the driving circuitry therefore and are characterized as emitting light by gas discharge with ion and electron recombination of a display medium, as including a system clock, the interruption of which causes an interruption in the presentation of the light emitting display, and as being capable of retaining display information upon the interruption of either a light sustaining signal or the electrical power as a result of an accumulation and retention of electrical charge across the display medium.
W. E. Johnson and M. J. Schmersal in their paper, A Quarter-Million Element AC Plasma Display With Memory, Society for information Display, 1971, International Symposium Digest for Technical Papers, May 197, discusses an example of a typical light emitting display which is appropriate for use with this invention. The light emitting display Model 80-33 Digivue (Trademark) Display/Memory Unit D223 by Owens Illinois Glass Company is appropriate for use with this invention and includes interface circuitry that accomodates an input light sustaining signal at a nominal frequency of 50 KH.sub.z with a voltage differential of about .+-.95 volts. While the light emitting displays themselves are capable of accepting a varying input frequency, the nature of the interface circuitry often limits the frequency range to about 30 KH.sub.z to 65 KH.sub.z.
2. Description of the Prior Art
Modulating the perceptible intensity of light can be effected by either directly or indirectly reducing the actual intensity of the light emitted or alternatively by periodically interrupting the emitted light and permitting the persistence of the vision of the human eye to perceive an averaged intensity of the emitted light.
One method which reduces the actual intensity of the light includes the use of an external apparatus to absorb or filter the emitted light. This light absorption may be accomplished by a simple mechanical neutral density filter or by an electrically controlled colloidal light valve as discussed by A. Davis, et al., in their paper Colloidal Light Valve (CLV) Digital Display, Society for Information Display, 1975, International Symposium Digest for Technical Papers, April 1975. While these filters reduce the intensity of each light pulse in an analogous manner and control the intensity from the maximum output to the lowest perceptible output without flicker, they are easily misaligned and include absorbing and reflecting surfaces which interfere with the maximum perceived intensity of the light source.
The alternative method, periodically interrupts the light source to reduce the perceptible intensity of the light, provided that the intensity of the light, the rate at which the light is turned off and on, and the duration of time that the light is off does not create a perceptible light flicker. Thus, for a given combination of conditions of light intensity, frequency of OFF and ON states, and duration of the ON state, the human eye will no longer average the intensity of the light, but will perceive a flicker or pulsation of light intensity.
Because of the nature of the display and drive circuitry used in conjunction with this invention, the generated light pulses are constant in both intensity and duration; that is, a light pulse is generated for appropriate transition, positive or negative, of a light sustaining signal, generally two light pulses for every full cycle of the light sustaining signal. Further, due to the nature of the display circuitry used with this display, the period of time that the light is on, the intensity of the light pulses, and the input frequency of the light sustaining signal are not adjustable. However, modulation of the perceived intensity of the light emitting display can be accomplished by controlling the number of cycles of the light sustaining signal over a given period of time.
Also, the technique of reducing the frequency of the light sustaining signal which is applied to the light emitting display has been used to modulate the intensity of the presentation of the light emitting display. However, due to the limitations imposed by the drive circuitry of the light emitting display, this method rarely yields more than a ten to one range in light intensities. (As discussed above, the conventional interface circuitry is generally limited to a range of about 30 KH.sub.z to 65 KH.sub.z.)
Another method for reducing the perceptible intensity of the light emitting display is achieved by altering the shape of the light sustaining signal. A number of different light emitting states have been found as a result of altering the wave form of the light sustaining signal. However, no more than three such stable states have been discovered and the majority of light emitting displays are capable of accepting only two such states. Accordingly, no more than three possible levels of light intensity have been developed by this approach.